As a result of the application of higher-order emission control standards to diesel engines, it has become necessary nowadays to install an exhaust gas treatment system in addition to a muffler on a vehicle body on which an engine is mounted. Examples of the above-mentioned exhaust gas treatment system include particulate matter trap filters, NOx treatment systems of the urea-selective reduction type, NOx catalysts, and the like. It is an NOx treatment system of the urea-selective reduction type that uses a liquid reducing agent for such reduction of NOx and the liquid reducing agent is an aqueous urea solution.
As a conventional technology of this sort, there is the technology described in Patent Document 1. In an exhaust gas treatment system according to this conventional technology, an oxidation catalyst, an NOx storage catalyst and a selective reducing-agent catalyst are arranged in an exhaust passage of an internal combustion engine. Further, the exhaust gas treatment system is provided with a reducing agent injection device for injecting a reducing agent, i.e., an aqueous urea solution, which is fed from an aqueous urea solution tank as a reducing agent storage tank, into the exhaust passage, that is, an injection nozzle, and also, with a remaining reducing-agent amount detection device for detecting a remaining amount of the aqueous urea solution in the aqueous urea solution tank, that is, an aqueous urea solution sensor.
This conventional technology is designed such that, when the remaining amount of the aqueous urea solution in the aqueous urea solution tank as detected by the aqueous urea solution sensor has become small, control is performed to reduce an output torque or the like and hence to lower the concentration of NOx in exhaust gas, thereby allowing the vehicle to reach a service station, where the aqueous urea solution can be replenished, while limiting the degree of decrease of the aqueous urea solution in the aqueous urea solution tank.
Patent Document 1: JP-A-2006-226171